The present invention relates generally to a stack damper control apparatus adapted for use with a damper in an exhaust stack of a fuel-burning temperature conditioning system. More particularly, the present invention relates to an apparatus for opening and closing a stack damper and coupling an energy source to a source of fuel in response to a signal from a temperature switch.
Fuel-burning temperature control systems such as gas fired furnaces require a vent stack to carry products of combustion from the burner. Vent stacks typically include a damper. When the burner is not in operation, the damper moves to a closed position in which the damper substantially closes off the vent stack to prevent heat loss. When the burner is in operation, the damper moves to an open position in which the vent stack is substantially clear. For safe operation, the damper must be in its open position before the burner is ignited. Accordingly, systems for controlling dampers typically include an interlock mechanism to ensure that the damper is fully open before the burner operates.
U S. Pat. No. 4,087,045, issued to Matthews on May 2, 1978 is representative of the prior art. It describes a system for controlling damper positioning which includes a safety interlock between the stack damper and fuel supply to prevent operation of the fuel supply whenever the damper is in other than its fully open position. The damper activates the interlock circuit by means of limit switches operated by cams located on the damper shaft. The interlock also prevents operation of the burner in the event of a failure of he control apparatus which may be caused by, for example, the welding of the switch contacts together or the shifting of the cams employed in cam-operated switches. The interlock also prevents lock out of the system during momentary power operation.
While the interlock described in Matthews generally provides a way of ensuring safe operation of a damper and fuel supply in a temperature conditioning system, it does exhibit several shortcomings. For example, it requires three relays for implementation, one of which being a three tap relay. The relays employed are expensive to use and require many circuit component connections. In addition, the cam-operated limit switches are of the type having lobes which deflect the contact arms of the limit switches. Such limit switches require calibration to assure that contact is made or broken at the appropriate angular location when the damper is in its fully open or fully closed position. Calibration of the limit switches adds to manufacturing cost. Also, no provision is made for any visually discernable indication of damper position which would be important during the performance of operational safety checks and maintenance on the damper actuator system. Further, modern safety standards require gas burning appliances to have a blocked vent shutoff system which automatically shuts off the fuel supply when a blocked vent condition is sensed. Matthews makes no provision for such a system.
Accordingly, a need exists for improvements in a damper actuator system which accommodates a blocked vent shutoff system as well as a visual indicator of damper position. Also, a need exists for a damper actuator system which is simpler and less costly to construct and which does not require calibration of limit switches.